PROJECT SUMMARY/ABSTRACT Obstructive sleep apnea (OSA) is a highly prevalent disorder with adverse neurocognitive and cardio-metabolic outcomes. Continuous positive airway pressure (CPAP) is the gold standard therapeutic option to treat airway obstructions during sleep and thus, prevent its adverse cardiovascular and neurocognitive outcomes. Previous clinical trials, however, have largely failed to show a consistent impact of CPAP on these health outcomes. One of the main limitations of these trials is, we believe, inadequate characterization of OSA and its acute physiological consequences. By characterizing OSA based on the ?apnea-hypopnea index (AHI)?, there is a potential risk of negative results. We surmise that, by better characterization of OSA-related physiological consequences during sleep, we will be able to identify individuals at high risk for these adverse outcomes and those who would benefit most from therapy. We have developed physiologically driven metrics to capture the precise burden of OSA-related hypoxemia (?hypoxic burden?), autonomic response (?heart rate burden?), and sleep fragmentation (?arousal burden?). Our preliminary data from large observational studies suggest that these metrics outperform conventional sleep study parameters. In Aim 1, we seek to demonstrate that OSA patients with high hypoxic burden will exhibit greater improvements, after 12 weeks of CPAP therapy, in endothelial function (flow-mediated vasodilation) and oxidative stress markers than those with a low hypoxic burden. In Aim 2, we will investigate how heart rate burden determines the reduction in 24-hour mean blood pressure after 12 weeks of CPAP treatment. Finally, in Aim 3, we will seek to demonstrate that OSA patients with larger degrees of sleep fragmentation, quantified by arousal burden, will respond more favorably to CPAP, in terms of improvement in daytime sleepiness and attention, than those with low arousal burden. While the primary analysis will be the change in these outcomes after 12 weeks of CPAP, we will also assess these outcomes at 4 weeks to examine their time course. A total of 158 men and women with apnea-hypopnea index ?15 events/hour will receive CPAP for 12 weeks. Adherence to therapy will be carefully monitored and encouraged by regular phone calls and in-person visits. Adverse events will also be closely monitored and recorded. Overall, our proposal is expected to demonstrate that prognostic markers of OSA that more strongly link with health outcomes will not only improve the diagnosis of OSA, but also provide a physiological basis for identifying those individuals most responsive to CPAP therapy. These results will have key mechanistic implications for ?individualized medicine? in OSA by focusing on subgroups of patients who would most benefit from CPAP therapy. This personalized medicine approach will provide the scientific knowledge needed to progress towards larger studies in selected patients. Such results are of major importance because they have great potential to improve the quality of life and health outcomes of patients with OSA.